It turns out that books bound in human skin, though uncommon, aren't exactly rare:

A number of prestigious libraries -- including Harvard University's -- have such books in their collections. While the idea of making leather from human skin seems bizarre and cruel today, it was not uncommon in centuries past, said Laura Hartman, a rare book cataloger at the National Library of Medicine in Maryland and author of a paper on the subject.

An article from the St. Louis Post-Dispatch from the late 1800s "suggests that it was common, but it also indicates it wasn't talked about in polite society," Hartman said.

Link to story | Boston Anthenaeum's book | Not to be outdone, Harvard has several of 'em.